As storage mediums for storing video information, voice information, or data including computer programs, various optical storage mediums have been conventionally proposed which include a so-called read-only optical disc, a phase-change optical disc, a magneto-optical disc, or an optical card.
Optical disc devices are used to write data on such optical storage mediums (hereinafter, referred to as “optical discs”) or read data recorded on optical discs. In the present specification, an optical disc device widely includes not only an optical disc drive but also various kinds of apparatuses capable of writing data on an optical disc and reading data from an optical disc. Namely, an “optical disc device” of the present specification includes, for example, a game machine, audio-visuals, a personal computer, and so on. Additionally, the optical disc device also includes a personal digital assistant (PDA) in which data can be written/read on/from a small optical disc.
Referring to FIG. 1, the configuration of the optical disc will be firstly discussed below. An optical disc 20 of FIG. 1 comprises, from the side irradiated with a light beam by the optical head, a substrate 21 made of a transparent material permitting the passage of a light beam, an information layer 29 for recording and reproducing data, and a protective layer 25 for protecting the disc. The substrate 21 also has a function of protecting data from a flaw or crack, contamination, and so on of a disk just like the protective layer 25. Besides, the “substrate” and the “protective layer” both indicate transparent members existing between the information layer of the optical disc and the atmosphere in the present specification. Therefore it is not necessary to distinguish between the “substrate” and the “protective layer” according to a material, a thickness, a manufacturing method thereof. Therefore, an optical head may be disposed on the side of the protective layer and a member represented as a “substrate” and a member represented as a “protective layer” may be replaced with each other in the present specification.
FIG. 2 is a perspective view schematically showing an enlarged information layer 29 of the optical disc 20. A light beam is emitted to the disc 20 from the upper side of FIG. 20. As shown in FIG. 2, convex tracks 28 are formed on the information layer 29 of the optical disc 20. The tracks 28 are formed concentrically or spirally with respect to the center of the disc. The tracks 28 may be wobbled. Information such as address information can be previously recorded on the optical disc 20 according to the wobbling shape and the wobbling frequency of the tracks 28.
FIG. 3 is a block diagram showing the configuration of a conventional optical disc device. The optical disc 20 is rotated by a disc motor 10 with a predetermined number of revolutions. A light beam emitted from a light source 3 such as a semiconductor laser, which acts as light beam irradiating means, is converged onto the information layer 29 of the optical disc 20 by an objective lens 1, which acts as converging means, and the light beam forms a light beam spot on a desired converging position on the information layer 29.
An optical system including the objective lens 1 is designed so that fixed spherical aberration correction is performed on the assumption that focus control is stably performed on the information layer 29 of the optical disc 20. Namely, optical design for minimizing spherical aberration is made according to the thickness of the substrate 21 of the optical disc 20. This is because dynamic correction is not necessary for spherical aberration in the conventional optical disc device.
Light reflected from the optical disc 20 is received by a light-receiving part 4 and photocurrent is generated according to a quantity of the received light.
The optical disc device comprises a focus actuator 2 and a tracking actuator 27. The focus actuator 2 moves the objective lens 1 substantially perpendicularly to the information layer 29 of the optical disc 20 to change the converging position of a light beam. The tracking actuator 27 moves the objective lens 1 in the radius direction of the optical disc 20 to permit the converging position of the light beam to correctly follow the tracks 28 on the information layer 29 of the optical disc 20.
The objective lens 1, the focus actuator 2, the light source 3, and the light-receiving part 4 are integrated into a module serving as an optical head 5. The optical head 5 can be moved in the radius direction of the optical disc 20 by a transfer table 60 acting as searching means. The transfer table 60 is driven by an output signal (driving signal) from a transfer table driving circuit 62.
Subsequently, focus control in the optical disc device will be discussed below.
A light beam generated by the light source 3 such as a semiconductor laser is converged on the information layer 29 of the optical disc 20 by the objective lens 1 and the light beam forms a light beam spot. Reflected light of the light beam spot from the optical disc 20 is inputted again to the light-receiving part 4 via the object lens 1.
The light-receiving part 4 is divided into four areas. Photocurrent is generated according to a light quantity detected in each of the areas and the photocurrent is outputted to a preamplifier 11. The preamplifier 11 comprises I/V converters. Photocurrent inputted from the light-receiving part 4 to the preamplifier 11 is converted into voltage by the I/V converters. Each converted signal is transmitted to a focus error signal generator 7 and a tracking error signal generator 18. The focus error signal generator 7 generates, from an output signal of the preamplifier 11, an error signal of the optical disc 20 and a light beam spot, which is outputted from the optical disc 5 and is focused, with respect to the vertical direction.
The optical system generally comprises a focus error detecting system using the astigmatic method and a tracking error detecting system using the push-pull method.
The focus error signal generator 7 generates a focus error signal (hereinafter, referred to as an FE signal) based on an input signal according to the astigmatic method. The FE signal, which is an output signal of the focus error signal generator 7, is subjected to a filtering operation such as phase compensation and gain compensation in the focus control section 17 and then the FE signal is outputted to a focus actuator driving circuit 9.
The objective lens 1 is driven by the focus actuator 2 based on a driving signal from the focus actuator driving circuit 9. As a result, the light beam spot is driven so as to have a predetermined converging state on the information layer 29 of the optical disc 20 and thus focus control is achieved.
The following will discuss tracking control in the optical disc device.
From an output signal of the preamplifier 11, the tracking error signal generator 18 generates, with respect to the radius direction of the optical disc 20, an error signal between the tracks 28 and a light beam spot which is outputted and focused from the optical head 5. The tracking error signal generator 18 generates a tracking error signal (hereinafter, referred to as a TE signal) based on an input signal according to the push-pull method. The TE signal, which is an output signal of the tracking error signal generator 18, is subjected to a filtering operation such as phase compensation and gain compensation in a tracking control section 19 and then the TE signal is outputted to a tracking actuator driving circuit 26.
The objective lens 1 is driven by a tracking actuator 27 based on a driving signal outputted from the tracking actuator driving circuit 26. As a result, the light beam spot is driven so as to follow the tracks 28 on the information layer 29 of the optical disc 20 and thus tracking control is achieved.
Referring to FIG. 4, the following will specifically describe the generation of the focus error signal and the tracking error signal.
As shown in FIG. 4, the light-receiving part 4 is divided into four areas A, B, C, and D. The areas A to D of the light-receiving part 4 generate photocurrent according to a light quantity detected in each of the areas and outputs the photocurrent to corresponding I/V converter 6a, I/V converter 6b, I/V converter 6c, and I/V converter 6d, which are included in the preamplifier 11.
Signals having been converted from current to voltage by the I/V converter 6a, the I/V converter 6b, the I/V converter 6c, and the I/V converter 6d are transmitted to the focus error signal generator 7 and the tracking error signal generator 18.
The “information track longitudinal direction” shown in FIG. 4 is a direction tangential to the tracks 28 of the optical disc 20, and the “optical disc radius direction” is a direction perpendicular to the tracks 28 of the optical disc 20. Therefore, in the focus error signal generator 7, the sum of the output of the I/V converter 6b and the output of the I/V converter 6d is subtracted from the sum of the output of the I/V converter 6a and the output of the I/V converter 6c, so that an FE signal is acquired by the astigmatic method.
In the tracking error signal generator 18, the sum of the output of the I/V converter 6b and the output of the I/V converter 6c is subtracted from the sum of the output of the I/V converter 6a and the output of the I/V converter 6d, so that a TE signal is acquired by the push-pull method.
In this way, the conventional optical disc device performs focus control and tracking control when information is written on the optical disc and/or information is read from the optical disc.
However, in the conventional optical disc device, it has become difficult to write/read information by using a high-density optical disc. This point will be discussed in detail.
In recent years an objective lens with a numerical aperture (NA) larger than 0.6 and a light source with a wavelength shorter than 650 nm have been proposed to further increase a recording density and a capacity of an optical disc. For example, a disc is proposed which has a numerical aperture of 0.85, a light source with a wavelength of 405 nm, a substrate (or a protective layer) with a thickness of 0.1 mm, and a capacity of 20 to 25 GB. Since a laser beam diameter (spot diameter) on the optical disc is proportionate to λ/NA, it is preferable to reduce λ and increase NA in view of improvement of a recording density, where λ represents a wavelength of a laser beam.
When NA is 0.85 and the light source has a wavelength of 405 nm, although a beam spot is reduced, the aberration of a light beam, particularly spherical aberration becomes too large to neglect. The spherical aberration is caused by the object lens and the substrate (or the protective layer) constituting the optical disc.
As shown in FIG. 1, the information layer 29 of the optical disc 20 is protected by the substrate 21. A light beam outputted from the optical head 5 passes through the substrate 21 and forms a light beam spot on the information layer 29.
In conventional DVDs using optical systems with an NA of 0.6, a change in spherical aberration caused by an uneven thickness of the substrate 21 is within a tolerance and thus the change is negligible. However, when the substrate 21 has an even thickness, the light beam spot has spherical aberration proportionate to the fourth power of the NA. Thus, when the NA is increased to 0.85, a change in spherical aberration becomes too large to neglect.
In a DVD standard, a double-layer disc having two information recording surfaces is also adopted to increase a recording capacity for each optical disc. FIG. 5 is a diagram showing an example of the configuration of the double-layer disc. As shown in FIG. 5, the double-layer disc comprises, from the side of an optical head, a substrate 21, an L0 layer (first information recording surface) 22, a spacer layer 24, an L1 layer (second information recording surface) 23, and a protective layer 25 on the back. The substrate 21 and the spacer layer 24 are composed of a transparent medium such as a resin.
According to the multi-layer structure of FIG. 5, on the optical disc 20 having more than one information recording surface, it is necessary to move the focal position of a light beam from the information recording surface, on which a light beam spot is currently positioned, to an adjacent information recording surface. Such a movement of the focal position of a light beam between the different information recording surfaces will be referred to as “interlayer movement” in the following description. Referring to FIGS. 3 and 6, the method of interlayer movement will be discussed below.
First, the following will describe the case where the focus of a light beam is moved from the information recording surface close to the objective lens 1 of the optical head 5 to the information recording surface far from the objective lens 1. A microcomputer 8 stops focus control once and simultaneously outputs, to the focus actuator driving circuit 9, an acceleration pulse for moving the objective lens 1. The acceleration pulse has a waveform of FIG. 6(a) and moves the objective lens 1 to the back (that is, to the information recording surface far from the objective lens 1).
Then, the microcomputer 8 compares a deceleration start level and an FE signal of the focus error signal generator 7. When the FE signal exceeds the deceleration start level, the microcomputer 8 outputs a deceleration pulse. When the output of the deceleration pulse is completed in the end, focus control is resumed.
The following will describe the case where the focus of a light beam is moved from the information recording surface far from the objective lens 1 of the optical head 5 to the information recording surface close to the objective lens 1. In this case, the acceleration pulse/deceleration pulse with the waveforms of FIG. 6(b) is applied by the same method, so that the focus of a light beam can be moved between layers.
A higher recording density and a larger capacity are also demanded regarding the double-layer disc. In order to meet such a demand, the numerical aperture of the objective lens needs to exceed 0.6 and the light source needs to have a wavelength shorter than 650 nm.
In the case of the double-layer disc, since the spacer layer 24 is provided between the L0 layer 22 and the L1 layer 23, regarding a thickness from the surface of the optical disc 20 on the side of the optical head to the information recording surface, the L1 layer 23 is larger in thickness than the L0 layer 22 by the thickness of the spacer layer 24. Such a difference in thickness causes spherical aberration. In an optical system of a DVD standard where the NA of the objective lens is 0.6, the spherical aberration is within a tolerance and thus information can be recorded and reproduced without correcting aberration. As described above, in the case where an objective lens having a larger NA (e.g., 0.8 or more) is used, when the objective lens is adjusted on one of the information recording surfaces, spherical aberration caused by the thickness of the spacer layer 24 on the other information recording surface cannot be negligible.
Namely, when the NA of the objective lens exceeds 0.6, the conventional optical disc device cannot record information or reproduce recorded information on both of the information recording surfaces.
When the NA exceeds 0.6 (e.g., to 0.8 or larger), the provision of a spherical aberration correction lens unit 15 in FIG. 7 can be considered. The spherical aberration correction lens unit 15 is typically composed of a pair of lenses. A relative distance between the pair of lenses is changed by moving one of the lenses. By using such a spherical aberration correction lens unit 15, when recording/reproduction are performed on the double-layer disc, it is possible to correct spherical aberration in a proper manner for the information recording surfaces, thereby eliminating spherical aberration caused by the spacer layer.
The spherical aberration correction lens unit 15 is driven by a plate spring. In this case, while quick response is achieved and control is performed with high accuracy, the spherical aberration correction lens unit 15 moves just in a narrow range and results in a narrow correctable range for spherical aberration. Particularly when an uneven thickness of the substrate, the uneven characteristics of the objective lens, and the uneven characteristics of the spherical aberration correction lens unit 15 are considered, the double-layer disc lacks a correction range, so that recording and reproduction cannot be performed in a proper manner.
In view of the above problems, an object of the present invention is to provide an optical disc device which is capable of stably recording or reproducing information even when an optical disc includes a substrate (or a protective layer) having an uneven thickness causing spherical aberration.
Another object of the present invention is to provide an optical disc device which performs spherical aberration control with quick response and a wide correction range for spherical aberration, even when the NA of the objective lens is increased more than the conventional NA (e.g., 0.8 or larger), so that recording/reproduction can be performed on a high-density and large-capacity optical disc.